The present invention relates generally to electro-optical displays of the type using electrochromic material (referred to as "ECD" hereinafter) which undergoes reversible color changes upon the application of an electric field. More particularly the present invention relates to an improved sandwich construction for an electrochromic display.
The conventional electrochromic display comprises a first glass substrate with one or more transparent electrodes and electrochromic layers thereon, a second glass substrate with a counter electrode, a seal member, and an electrolyte filled therebetween. The electrolyte has a tendency to damage the glass substrates of the ECD cell in response to a high temperature applied to the ECD cell because the cubic expansion of the electrolyte is much greater than that of the glass substrates. To avoid the above destruction of the ECD cell, the present inventors have proposed an improved ECD, wherein a bubble comprising an atmosphere or gas is introduced into the electrolyte of the ECD cell to thereby cancel the cubic expansion of the electrolyte. The bubble is selected such that its volume is larger than the cubic expansion of the electrolyte.
However, the bubble tends to freely shift or move within the ECD cell. The optical characteristics of the ECD when the bubble attaches to either the electrochromic layers or the counter electrode since electrochemical reactions near the electrochromic layer and the counter electrode are prevented because of the near proximity of the disagreeably settled bubble to the electrochromic layer and counter electrode. When the bubble comes into contact with the first glass substrate, the optical indication characteristics of the ECD is also lowered because a white background included in the electrolyte becomes non-uniform because of by the bubble.
A porous film of pigment solidified with adhesive is disclosed in U.S. Pat. Nos. 3,892,492 and 3,994,333. The porous film functions to provide a background for a visual display and hides the counter electrode. However, the electrolyte is continuously filled within the ECD cell and the remaining electrolyte, except for that which is contained in the porous film, is not eliminated from the cavity.